U.S. patent number 5,254,149 [Application Number 07/864,279] was granted by the patent office on 1993-10-19 for process for determining the quality of temper of a glass sheet using a laser beam.
This patent grant is currently assigned to Ford Motor Company. Invention is credited to Amin H. Hashemi, Timothy L. Kelly.
United States Patent |
5,254,149 |
Hashemi , et al. |
October 19, 1993 |
Process for determining the quality of temper of a glass sheet
using a laser beam
Abstract
The quality of temper in a glass sheet is determined by
repeatedly scoring the glass sheet with a laser beam, to
incrementally penetrate the compression zone, until the glass sheet
shatters.
Inventors: |
Hashemi; Amin H. (Farmington,
MI), Kelly; Timothy L. (Ann Arbor, MI) |
Assignee: |
Ford Motor Company (Dearborn,
MI)
|
Family
ID: |
25342908 |
Appl.
No.: |
07/864,279 |
Filed: |
April 6, 1992 |
Current U.S.
Class: |
65/29.18;
73/760 |
Current CPC
Class: |
C03B
33/082 (20130101); C03B 27/00 (20130101) |
Current International
Class: |
C03B
27/00 (20060101); C03B 33/08 (20060101); C03B
33/00 (20060101); C03B 032/00 () |
Field of
Search: |
;65/29
;73/760,159,788,799 ;219/121.67,121.68,121.69 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Jones; W. Gary
Assistant Examiner: Griffin; Steven P.
Attorney, Agent or Firm: Melotik; Lorraine S. May; Roger
L.
Claims
What is claimed is:
1. a process for determining temper quality in a glass sheet,
comprising the steps of:
A) scoring a major surface of a tempered glass sheet with a laser
beam; and
B) repeating step A a number of times sufficient to cause the glass
sheet to shatter.
2. The process for determining temper quality in a glass sheet
according to claim 1, wherein the laser beam is stationary with
respect to the glass sheet.
3. The process for determining temper quality in a glass sheet
according to claim 1, wherein the laser beam is moved in a
direction parallel to the major surface of the glass sheet.
4. The process for determining temper quality in a glass sheet
according to claim 1, wherein the laser beam has an output energy
level from about 10 watts to about 30 watts during each
scoring.
5. The process for determining temper quality in a glass sheet
according to claim 1, wherein the laser beam has output energy
levels during at least two of the scorings which are different.
6. The process for determining temper quality in a glass sheet
according to claim 1, wherein at least two time intervals between
the scorings are different.
7. A process for determining temper quality in a glass sheet,
comprising the steps of:
A) scoring a major surface of a tempered glass sheet a first number
of times with a laser beam; and
B) scoring the major surface of the tempered glass sheet with a
laser beam a second number of times sufficient to cause the glass
sheet to shatter;
wherein time intervals between the scorings of step A are shorter
than the time intervals between the scorings of step B.
8. The process for determining temper quality in a glass sheet
according to claim 7, wherein the laser beam is stationary with
respect to the glass sheet.
9. The process for determining temper quality in a glass sheet
according to claim 7, wherein the laser beam is moved in a
direction parallel to the major surface of the glass sheet.
10. The process for determining temper quality in a glass sheet
according to claim 7, wherein the laser beam has an energy output
level from about 10 watts to about 80 watts during each
scoring.
11. The process for determining temper quality in a glass sheet
according to claim 7, wherein the laser beam has output energy
levels during at least two of the scorings which are different.
Description
FIELD OF THE INVENTION
This invention is directed to a process for determining the quality
of temper of a glass sheet. More particularly, the invention is
directed to a process for determining certain characteristics of
the compression and tension zones of a tempered glass sheet, and
thereby the quality of temper of the glass sheet.
BACKGROUND OF THE INVENTION
It is well-known that ceramic materials are much stronger in
compression than in tension. Therefore, "tempered" glass is
typically used for glass doors, vehicle glazings, and other
high-strength-requirement applications. Residual compressive
stresses are intentionally induced in tempered glass by heating a
glass sheet to a temperature near its softening point, removing it
from the heating furnace, and quickly directing blasts of a cooling
fluid, such as air, toward the major surfaces of the glass sheet.
The surface regions of the glass sheet contract because of the drop
in temperature as a result of convective heat transfer to the
cooling air. Thus, the major surface regions of the glass sheet
become rigid, while the central portion of the glass sheet remains
hot and can adjust its dimensions to the surface region
contractions. When the central region of the glass sheet cools and
contracts slightly at a later time, compressive stresses are
produced in the major surface regions of the glass sheet. A
constant cooling rate applied to both major surfaces of the glass
sheet, resulting from an identical flow of constant-temperature
cooling air to both major surfaces, theoretically would produce a
parabolic stress distribution when measured normal to the major
surfaces of the glass sheet.
Tempered glass is particularly useful for high-strength-requirement
applications because the exposed surfaces of the tempered glass
sheet are under residual compressive stress. Glass failure usually
occurs under an applied tensile (rather than compressive) stress.
Therefore, since failure in a glass sheet almost always is
initiated at one of its major surfaces, e.g., by striking the glass
sheet, any applied stress must first overcome the residual
compression near the surface of the glass sheet before that region
is brought into tension such that failure may occur.
It is generally known in the art of manufacturing automotive
glazings to heat a glass sheet templet to a temperature above its
plastic set temperature, usually about 1200.degree. F., then form
the templet to a desired curvature by either gravity forming or
press bending the hot glass, and thereafter temper the formed glass
templet by directing streams of a tempering fluid, usually moist
air, against the major surfaces thereof. During the tempering
operation, it is known to support the formed glass sheet on a
support ring, comprising a rigid structure conforming generally in
outline and elevation to the underside peripheral marginal surface
of the formed glass sheet.
During the tempering operation, the blasts of tempering fluid
rapidly cool the major surfaces of the formed glass sheet in all
areas, except those areas near points of contact between the
tempering support ring and the underside peripheral marginal
surface of the glass sheet. In those areas, cooling is retarded due
to the restricted flow of tempering fluid caused by interference
with the tempering support ring. Thus, those areas of the
ultimately produced glass sheet may be stressed in tension while
the majority of the major surface area of the glass sheet is
stressed in compression. This stress imbalance often leads to
spontaneous breakage of the glass sheet during use in a motor
vehicle.
Moreover, other variables in the tempering process can result in
poor quality, nonuniform tempering, wherein the configuration of
the actual stress distribution measured across the thickness of the
glass sheet at any Point along the surface of the glass sheet
varies markedly from an idealized parabola. Clearly, the quality of
temper induced into a glass sheet is difficult to control, and even
more difficult to measure quantitatively.
It would be desirable to devise a method for determining the
quality of the temper induced into a glass sheet. Such a method
should be simple, and yet give consistent, reproducible results
which could be used to control the tempering process and therefore
the quality of the tempered glass sheet produced thereby.
SUMMARY OF THE INVENTION
Accordant with the present invention, there surprisingly has been
discovered a process for determining the quality of temper of a
glass sheet. The process comprises the steps of:
A) scoring a major surface of a tempered glass sheet with a laser
beam; and
B) repeating step A a number of times sufficient to cause the glass
sheet to shatter.
The process of the present invention is particularly well-suited
for determining the quality of temper in an automotive glazing,
which information can then be used as a quality control tool for
modifying the operating conditions of the tempering process.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
The present invention is directed to a process for determining the
quality of tempering of a glass sheet. A surface of the glass sheet
is scored by a laser beam set at a predetermined energy level. This
scoring process is repeated a number of times (i.e., a number of
scoring "passes" are made), until the glass sheet shatters. Data
regarding the energy level of the laser beam, the number of times
the glass sheet is scored, and the time interval between each
scoring treatment may then be compared to an established matrix of
empirical data which would indicate the quality of the temper.
The quality of temper in part is determined by several factors,
including the thicknesses of the compression zones of the two major
surfaces of the glass sheet relative to the thickness of the
central tension zone, the thicknesses of the compression zones
relative to each other, the uniformity of thickness of either or
both of the compression zones, the average and maximum compressive
and tensile stress values of the tempered glass sheet (usually
expressed in psi), etc. A major concern in the vehicle glazing
manufacturing industry is the uniformity of the compressive stress
in the underside peripheral marginal surface compression zone where
the glass sheet engages the tempering support ring. The compressive
stress in this region often is very low due to the lack of an
adequate flow of tempering fluid adjacent the underside peripheral
marginal surface of the glass sheet caused by interference with the
tempering support ring. The present invention allows a quantitative
measurement of the stress induced into the glass sheet in this and
other areas of the glass sheet.
A surface of the glass sheet is scored by a laser beam set at a
predetermined energy level. By the term "laser", as it is used
herein, it is meant a device capable of "light amplification by
stimulated emission of radiation." Lasers are well-known in the
art, and therefore will not be discussed in detail. As will be
readily apparent to one ordinarily skilled in the art, the laser
must be selected to produce an energy beam at a wavelength which is
absorbed by the glass sheet. Useful wavelengths include, without
limitation, those from about 0.1 to about 1,000 microns.
Conveniently, the energy output for the laser may range over wide
limits from about 10 watts to about 300 watts. Preferably, the
energy output for use according to the present invention may range
from about 10 watts to about 80 watts.
The inventive process requires that the surface of the glass sheet
be scored a number of times. It is contemplated that the laser
output energy may remain constant each time the glass sheet is
scored, or may be set to a new value each time the glass sheet is
scored, or may comprise any combination of constant and varying
values. For example, the inventive method may include scoring the
glass sheet using 30 pulses of constant-energy laser light, with a
dwell time between pulses of 200 milliseconds, followed by a number
of intermittent pulses of laser light having a lower energy level
wherein the dwell time between each such intermittent pulse is
about one minute. It also will be apparent to one ordinarily
skilled in the art that the time intervals between each scoring
pass may be constant, varied, or any combination thereof.
The surface of the glass sheet is scored by the laser a number of
times, according to the present invention. By the term "score", as
used herein, is meant a process whereby a small amount of the glass
is obliterated by the laser energy. Thus, a bore or groove
increasingly penetrates the surface of the glass sheet during each
scoring pass. In the case where the laser beam remains motionless
relative to the glass sheet during each scoring pass, a bore
results in the glass surface. Where the laser beam is moved in a
direction parallel to a major surface of the glass sheet, a groove
is formed in the surface of the sheet. The terminology "laser beam
is moved in a direction parallel to the surface of the glass sheet"
is intended to include not only movement of the laser beam with
respect to the glass sheet, but also movement of the glass sheet
with respect to the laser beam, and any combination thereof. Thus,
this terminology is intended generally to encompass relative
movement between the laser beam and the glass sheet so as to result
in the formation of a groove in the surface of the glass sheet. The
bore or groove becomes deeper upon each scoring pass. By
comparison, when a constant beam of laser energy is directed toward
the surface of a glass sheet, as is conventionally done when laser
beams are used to cut glass sheets into smaller pieces, a hole is
drilled completely through the glass sheet, and the glass sheet
then shatters when the perforated glass sheet cools.
Successive scoring passes cause the bore or groove to penetrate the
compression zone of the exposed major surface of the glass sheet.
While not wishing to be bound by any particular theory regarding
the mechanism by which the scoring passes of the present invention
cause the tempered glass sheet to shatter, it is believed that the
scoring passes cause the glass sheet to shatter spontaneously when
the bore or groove has penetrated the glass sheet to the interface
between the compression and tensile zones. Therefore, the present
invention could be used to determine empirically the depth of the
compression zone at any location across the surface of the tempered
glass sheet.
It is contemplated that the data derived from the method of the
present invention may be compared to an established matrix of
empirical data which would indicate the quality of the temper. For
example, a large number of glass sheets which are identically
tempered could be split into two groups. The first group could be
subjected to the method of the present invention, to determine a
set of conditions under which the tempered samples spontaneously
shatter. The second group could be subjected to well-known tests
for determining the acceptance or failure of tempered glass sheets
based upon impact resistance, such as the tests set forth in the
American National Standard Institute (ANSI) Safety Code for Safety
Glazing Materials for Glazing Motor Vehicles Operating on Land
Highways, No. Z26.1. Thereafter, other sets of test groups of
tempered glass sheets produced under various tempering conditions
could be made and tested as above, to form a matrix of empirical
data which could then be used to indicate which glass sheets
subjected to the destructive testing method of the present
invention would have passed the ANSI Code. Utilizing this
knowledge, the tempering conditions or tempering support ring
materials and configuration could be modified accordingly, to
produce a tempered glass sheet of high quality.
EXAMPLES
Three samples of identically tempered 1/4 inch glass were subjected
to identical scoring passes from a stationary laser. The laser beam
was set at a power level of about 60 watts. The surface of each
sample was initially scored with 30 pulses of energy, each pulse
lasting 1 millisecond, with a dwell time of 200 milliseconds
between each pulse. Thereafter, additional intermittent pulses were
used to further score the glass sheet at one-minute intervals. The
laser output energy was maintained at 60 watts, and each pulse was
1 millesecond in duration. Samples 1 and 2 each shattered following
the 18th additional pulse; sample 3 shattered following the 17th
additional pulse.
This example may be repeated with similar success by substituting
the generically or specifically described test conditions recited
herein for the ones actually used in the preceding Examples.
Surprisingly, the present method provides consistent and repeatable
indications of the quality of temper of the glass sheet.
From the foregoing description, one ordinarily skilled in the art
can easily ascertain the essential characteristics of this
invention, and without departing from the spirit and scope thereof,
can make various changes and modifications to the invention to
adapt it to various usages an conditions.
* * * * *